1. Field of the Invention
This invention relates generally to the detection and quantification of certain earth formation elements surrounding a borehole, and especially elements sensitive to the nuclear activation reaction.
2. Description of the related art
The detection and determination of certain elements present in an earth formation or in the well itself is of great importance in the oil production business, since the presence and amount of these elements provide useful information. Among the logging techniques used for such determination are the nuclear logging techniques, and particularly the nuclear activation method wherein a sonde comprising a high energy neutron source and a gamma ray detector is lowered in the well to investigate. Certain atoms interact with neutrons in being activated to an unstable state which decays back exponentially in time to a stable state, while emitting gamma rays of given energy representative of the activated atom. Count rates in the gamma ray detector are proportional to the total amount of the element of interest present around the sonde. By way of example, oxygen, silicon, aluminum, magnesium, or gold atoms may be activated.
Oxygen atoms are representative of water. For example, a well which has been determined to be promising for oil production, is provided with a metallic casing, and cement is injected between the earth formation and the casing. Perforations are then made through the casing/cement and in the oil productive formation, so as to allow oil to flow up to the surface through a tubing beforehand arranged in the well coaxially to the casing. Unwanted vertical flow of water can occur in the cement, between the casing and the formation. This phenomenon, usually called "water channeling", causes undesirable paths between formations located at different depths, i.e. at different pressures, e.g. by allowing water from a first formation layer to mix with oil coming from a second formation layer. This phenomenon disturbs substantially the oil production. It is of great importance to identify fluid points of entrance to or exit from the borehole, as well as to determine the mechanical integrity of the cement annulus.
The invention may also be used, besides the water flow detection hereabove referred to, to qualify gravel packing which is usually disposed in the annulus between the productive formation and the screened tubing, for preventing sand invasion in the tubing. As a matter of fact, aluminum and silicon are found in abundance in gravel-pack material. Aluminum e.g. is found in sintered bauxite and silicon in standard gravel pack. The use of the activation method for the detection of aluminum has been depicted in the article "The Aluminum Activation Log" from H. D. Scott and M. P. Smith, in The Log Analyst, Sep.-Oct. 1973, pages 3-12.
Magnesium is another element of interest which is sensitive to the activation reaction. Magnesium is indicative of the presence of shales.
Moreover, gold atoms, when bombarded with high energy neutrons, become activated. It is known to inject gold as a tracer in oil wells. Activated gold follows the fluid paths in the formation and thus, presence of gold is thus representative of fractures in the formations.
Accordingly, there is a need for a better knowledge relating to the above mentioned elements in the earth formation.
However, the known methods and apparatus, although satisfactory, show limitations.
First, they do not offer a straightforward and simple way to distinguish the different activated elements one from the other. As a matter of fact, the gamma rays received by the detector may come from different types of activated atoms.
Furthermore, the known methods do not generally provide any quick and reliable information about the radial distance between the activated elements and the borehole.
According to the above, there is a need for a reliable method for obtaining quantitative and qualitative information related to given earth formation elements able to be activated by neutrons.